Mixed ultimate strategy is really no different from strategy in Open or Women's ultimate. The ability to read and adjust to a given game is very important. Develop a few strategies that your team is good at, and then learn how to read the game and react to it.
On offense, the key is recognizing your players' strengths. Outside of the huck game (which favors men receiving), your players will dictate their positions. And getting your players comfortable in all positions is critical.
Spacing is essential in all ultimate—some would argue even more so in Mixed. It's important to make all your players effective, and that requires space.
On 6 Trained Monkeys, on defense, we would primarily force backhand. For two reasons: first was that the majority of the teams played primarily a forehand mark. While this isn't true as much of the best teams, it is what most of the teams were doing. We wanted to be different when we faced the good teams. The second reason we went force backhand was that the teams we played in the region were significantly better at breaking a forehand mark (probably because of the first reason).
This strategy was effective all the way through the UPA Championships. We only changed from this for reasons that would apply in any game: strong cross wind, a big huck game from our opponent that favored backhands, or sometimes just to screw with stuff. Even then we changed to create confusion, but not to go away from our own strengths. We would used a straight-up force for a certain number of throws to confuse or to attack those teams that either had focused plays or used a horizontal stack. This really was just a variation as it almost always went back to force backhand.
Against the better Mixed teams, we really focused on stepping up the intensity on the mark. I've seen this from other Mixed teams, and you see it through the top levels of the other divisions. I don't think this is unique to Mixed, and I think that it really was the most critical part of success against better teams.
The skills that are most important in Mixed are, as well, no different from single-gender ultimate: poise, smart choices, intensity on D. Someone who can break the mark doesn't stop being able to because of who the receiver is. Someone who can put on a great mark can keep doing it in Mixed. In Mixed ultimate, physical skills are not the deciding factor, either. I think that the key is having players of both genders who can fit into all of the positions.
The psychology of genders is a huge topic. It's easy on Mixed teams of any level to have the men dominate the huddle, dominate the touches of the disc, dominate the decision making. The best teams will have strong players of both genders, in all positions (on and off the field), to allow for flexible and smart adjustments to a variety of skills from the opponent. This has to be part of the make-up of any successful Mixed team.
Of course, having a woman who can send it helps, too.